Lachlan Macquarie's cousin. Son of Murdoch Maclaine (1736-1804), the 19th laird of Lochbuie, and Jane [neeCampbell] (c.1765-1824) [sister of Elizabeth Macquarie]. It is this latter association - through his wife, Elizabeth, rather than his uncle Murdoch - that accounts for Macquarie referring to Maclaine as his 'nephew' rather than as his 'cousin'.
In September 1801 Macquarie arranged ensigncies for two young relatives: his cousin, John, and his nephew, Hector Macquarie (despite the fact that both boys were under the age of 10). However the subterfuge was found out by the War Office in 1804 and Macquarie was lucky to escape possible discharge from the army. He was severely censured and it damaged his reputation with the Duke of York, Commander-in-Chief of the Army. In the short term it certainly ruined any hopes Macquarie may have had for remaining in England for a longer period - and he was ordered to rejoin his regiment in India in April 1805.
In 1808, Macquarie again tried to acquire an ensigncy for John Maclaine (now aged 16) in several regiments and eventually found one for him in his own regiment, the 73rd. As a consequence he accompanied Macquarie in his posting to NSW on board the Dromedary in 1809.
He remained a constant member of the governor's inner circle of associates and accompanied the Macquaries during their visits to Van Diemen's Land and Newcastle in 1811-1812. When Captain Henry Antill was promoted to brigade-major in 1812 Macquarie appointed Maclaine as his new aide-de-camp. However the young man's intemperate behaviour and impulsiveness were constant sources of embarrassment to the governor. Firstly, Macquarie was forced to pay £300 to clear Maclaine's debts; and then, in February 1813, Maclaine broke his arm when he fell from the mast of a ship during a drunken party.
When the 46th Regiment arrived in 1814 to replace the 73rd and his regiment was posted for duty in Ceylon, Lieut. Maclaine departed for England on board the brig James Hay [on 2 June 1814] carrying Macquarie's despatches to Lord Bathurst. By July 1815 he was in Paris celebrating the defeat of Napoleon's army at Waterloo on 18 June.
He embarked on board the Wellington for Ceylon (from Gravesend) on 19 September 1816 with three other officers of the 73rd Regiment: Major James Vallance, Captain Loftus Owen and Ensign Mark Lidwill.
John Maclaine was killed in action on 13 January 1818 while leading (on horseback) a detachment of men of the 73rd Regiment through heavy jungle terrain in Ceylon. He made an easy target for the Kandyan opponents concealed in the dense foliage and died instantly from a single shot. His commanding officer, Lt. Col. Maurice O'Connell, clearly believed that he was a victim of his own folly in disregarding the advice of his men not to expose himself to enemy fire.